Noble metals such as the platinum group metals are in general use as catalysts for automotive exhaust gas purification, organic chemical reactions, as well as fuel cell electrodes. They are also constituents of electronic parts. The recovery of noble metals from used materials is important due to their rarity and expense.
This has hitherto been by the dissolution process. In this process, the metals are dissolved with the carriers in hot concentrated sulfuric acid or an oxidizing acid mixture such as aqua regia, and the metals are separated from the solution by addition of a reducing agent or by electrolyzing the solution in low current density electrolytic cell. In the former case, the noble metals separate out in the solution, while in the latter case they are deposited on the cathode.
The dissolution process requires a hazardous, highly oxidizing acid that is difficult to handle, and a large amount of heat to maintain the acid temperature. Additionally, aqua regia, long been used for noble metals dissolution, gives off NOx producing pollution treatment problems. Cyanides, such as sodium cyanide, are effective for noble metals dissolution, but need careful handling and adequate liquid waste treatment due to their high toxicity.
In addition, the dissolution process is uneconomical in noble metals recovery from noble metal catalysts, because it involves the treatment of a large amount of carrier containing solutions as well as noble metals. (Noble metal catalysts are composed of carriers such as carbon, silica and alumina and very small amounts of noble metals therein. Moreover, at times a portion of noble metal is present as oxides, barely soluble in aqua regia. In such cases, it is necessary to reduce the noble metal oxides before dissolution, or perform the dissolution process in two steps, i.e., noble metal dissolution in acid and noble metal oxide dissolution in alkali.
Fuel Cells are devices that release electrical energy using an electrochemical reaction. A major class of fuel cells utilizes hydrogen fuel, and the electrochemical oxidation of hydrogen as well as the electrochemical reduction of oxygen to water is catalyzed using electrodes containing precious metal catalysts. Precious metal catalytic elements for use in precious metal catalysts include, but are not limited to, platinum (Pt), ruthenium (Ru), palladium (Pd), gold (Au), and rhodium (Rh). It is widely accepted that the high cost and limited supply of platinum and other catalytic elements are obstacles to large scale commercialization of fuel cells.
There are several types of fuel cells. Most common is the polymer electrolyte membrane (PEM) fuel cell. The PEM forms the basis for a membrane electrode assembly (MEA), which is the structure by which hydrogen can be oxidized to generate electricity. An anode (i.e., a negative electrode) is provided on one side of the PEM and a cathode (i.e., a positive electrode) is provided on the opposite side of the PEM. The anode contains a catalyst, typically comprising platinum, for promoting dissociation of hydrogen into electrons and positive hydrogen ions. The cathode also contains a catalyst, typically comprising platinum, for promoting reduction of oxygen. An MEA typically carries a catalytic element loading in the order of a few mg/cm2. Typically, these loadings in current commercial fuel cells translate to about 0.5% to 2.0% by weight of catalytic element in the MEA.
A commonly used polymer electrode membrane is Nafion™ by E.I. DuPont de Nemours Company. Nafion™, a Teflon™-based polymer, is a sulfonated perfluropolymer. Even when using a membrane that is itself free of fluorine, a perfluropolymer ionomer is typically mixed into the electrocatalyst layers (i.e., the anode and the cathode) to improve the mobility of the positive hydrogen ions. Additionally, the presence of the perfluorinated polymer makes the powder of the MEA hydrophobic when the MEA is ground.
In one type of fuel cell, the anode and cathode are coated onto the PEM to form a catalyst coated membrane (CCM). A CCM fuel cell can include platinum, ruthenium, and other catalytic elements. In another type of fuel cell, the anode and cathode catalyst materials are coated onto their respective carbonaceous gas diffusion layers toto form gas diffusion electrodes (GDE), which are then sandwiched around the PEM. A GDE fuel cell can also include platinum, ruthenium, and other catalytic elements. The gas diffusion layers provide for the uniform distribution of hydrogen and oxygen to their respective sides of the PEM, provide a conductive pathway for electricity to be transmitted out of the fuel cell, and provide a porous means for the water produced by the electrochemical reaction to be transported away.
Another type of fuel cell using catalytic elements such as platinum is an alkaline fuel cell (AFC). Still another type of fuel cell using catalysts is a phosphoric acid fuel cells (PAFC), which use a poly-benzylimidazole (PBI) membrane saturated with phosphoric acid electrolyte. Regardless of the type, after a period of use, a fuel cell often must be replaced, due to fouling of the catalyst, or for another reason. In particular, after repeated cycling of the fuel cell during operation (i.e., cycling between periods of high and low voltage generation), the catalyst can tend to migrate into the membrane and the catalytic element particles can become altered in size and therefore less effective. Rather than simply disposing of a fuel cell that must be replaced, it is highly desirable to recover as much catalytic element as possible from the MEA, due to the value of the precious metal.
The conventional approach to recover platinum and other precious metal catalytic elements from an MEA includes combusting the PEM and the carbonaceous diffusion layers, dissolving the resultant ash in acid, and purifying the precious metal using standard refining chemistry. However, the high fluorine content of the MEA, particularly those with Nafion™ or other Teflon™-based membranes, results in toxic emissions of hydrogen fluoride gas (HF) and other fluorine compounds from the combustion process.
To dissolve and recover the platinum catalyst, the majority of available methods use hydrochloric acid leaching media assisted by a chemical oxidant such as nitric acid (i.e. aqua regia), halogen gas (e.g. Cl2, Br2), and even hydrogen peroxide [1,2]. The platinum chloro complexes thus obtained are subsequently extracted by solvent extraction processing, stripped from the organic phase by water and possibly precipitated as (NH4)2PtCl6 in fairly high purity (99.95%) by addition of ammonium chloride to the aqueous phase [3]. Possible emissions to air from this type of processing include ammonia, chlorine, hydrogen chloride and nitrogen dioxide. The described industrial process is, seemingly, designed with the intention of recovering the noble metal catalyst as pure salts that can be traded. This line of thought is also followed in platinum catalyst recycling schemes specifically considered for polymer electrolyte membrane fuel cells [4-8].
Several methods exist for the preparation of supported and unsupported noble metal catalysts based on these commercially available salts [9].
EP0363314A1 describes a method for recovery of precious metals such as platinum, palladium and rhodium used as catalysts. The method comprises dissolving the metals by placing them in a solution comprising an electrolyte such as hydrochloric acid and electrolysing the solution. The pure metals are precipitated by adding a reducing agent to the solution. EP0363314A1 does not disclose an integrated process for obtaining the metals in a catalytic structure.
There is a need for a more simple method for recovery of precious metals in catalytic structures.